1. Field of the Invention
The present invention relates to seals employed in elastic fluid axial flow turbines and, more particularly, to segmented packing ring seals arranged both where rotatable shafts penetrate stationary turbine casings and, in addition, internal to the casings between stages and turbine sections.
2. Description of the Prior Art
Generally, such known seals prevent or reduce leakage of the fluid by creating small clearance areas with low flow coefficients between the rotating and stationary parts. Improved efficiency, minimized loss of fluid and prevention of undesirable side effects caused by leakage of fluid are objectives of such seals.
Also, these segmented, labyrinth type seals are vulnerable to rubbing damage caused by turbine misalignment, vibration and thermal distortion. Most of these damage causing factors are more likely to occur during starting, at light loads or following sudden loss of load. As a result, not only do the seals become worn, but at the point of rubbing, heat is generated in the shaft causing bowing of the shaft with resultant vibration and increased damage to additional shaft packing seals and the even more critical tip seals that limit leakage past rotating blades.
Traditional segmented packing rings employ springs that force the segments toward the shaft as limited by shoulders on the packing ring holder. The springs are normally designed to provide assurance that all segments will be pushed inward until limited by either the shaft or the shoulders. The lowest segment requires the greatest force, since the weight of all higher segments must often be supported by the lowest one, thus requiring a spring force equal to the weight of all segments, plus some safety margin for unavoidable friction. Since each segment normally has the same spring behind it, the force that must be overcome by the shaft to make any of the packing segments shift positions is quite large, with the unavoidable ability to generate large amounts of local heat in the shaft.
U.S. Pat. No. 4,436,311 issued to Ronald E. Brandon describes retractable packing rings which, during start-up conditions, have large radial clearance that automatically decreases to a small clearance condition when a predetermined flow condition has been reached. Such rings have been successfully applied in turbine applications where the pressure forces are significantly greater than the weight forces of the packing segments. In the low pressure stages, however, weight forces are often too large to be successfully overcome by available turbine pressure forces.
In U.S. Pat. No. 5,395,124 issued to Ronald E. Brandon there is disclosed a segmented, retractable, labyrinth-type shaft sealing system for low pressure turbines wherein the packing segments are provided with a combination of gravity or levitation springs that provide a vertical force to neutralize segment weight forces and butt springs that resist pressure forces so as to cause a large clearance at turbine start up and then permit segment motion to allow the clearance to become small after operational conditions are established.
Retractable designs, however, cannot be readily applied to several turbine packing locations. These include, but are not limited to, the outer three packing rings in each turbine casing and double flow nozzle diaphragms.
With regard to the three outer rings it has not been possible to use retractable rings in these turbine locations due to the fact that they must be in the closed position during start up, and also, they must operate for the most part with the same pressure drop at all operating conditions.
As originally designed, these packing rings must employ relatively strong back springs that support the segments and simultaneously force them toward the shaft, yet during periods of shaft misalignment when the packing is rubbing the shaft, allow the segments to yield. Because of the relatively high spring force, the rubbing contact of shaft and packing teeth generates significant amounts of heat in the shaft, leading to bowing, rubbed packing teeth and tip seals, vibration and aborted start ups. The thermodynamic losses are significant.
The heat generated during such events is proportional to the spring force that resists the temporary motion of the packing segment when the teeth are in contact with the rotating shaft. It is thus desirable to minimize the spring force selected to keep the segments in the closed position. It is expected that this will reduce heat generation to less than one-third that experienced in original designs.
In some intermediate pressure turbine sections and almost all low pressure turbine sections, a double flow entry nozzle is provided. This nozzle needs a packing ring to prevent leakage along the shaft from one end of the nozzle to the other. This is due to the fact that the pressure never turns out to be exactly the same at both sides. Leakage effects can be serious since a 1% leak will cause a 1% stage loss at each end of the nozzle.
These segments cannot be made retractable since there is no designated pressure difference on which to base closing forces. As a result, when being rubbed by the shaft these rings can cause significant vibration and shaft bowing. The undesirable effects are magnified by the fact that the double flow nozzle is highly likely to distort during both transient and steady state operation, and, worse, being located in the center of the span, to cause severe bowing of the shaft.
In both of the above cases it would be desirable to reduce the spring forces, yet still assure a closing motion of packing segments when the rubbing condition is over. That is the purpose of this invention.